1. Field of the Invention
The present invention relates to a wet electro-photographic printer using a liquid developing agent, and more particularly, to a wet electro-photographic printer which transfers and/or fixes an image formed on a transfer belt to and/or on a sheet of printing paper via a subsidiary intermediate transfer unit to improve a transfer efficiency instead of transferring the image on the transfer belt directly to the printing paper.
2. Description of the Related Art
A conventional electro-photographic printer creates an image by transferring and fixing an image to and on a sheet of printing paper after forming an electrostatic latent image on a photosensitive medium, such as a photosensitive belt or a photosensitive drum, and developing the electrostatic latent image using developing agents with predetermined colors. The electro-photographic printer is divided into a wet type and a dry type according to developing agents. The wet electro-photographic printer uses a developing liquid having a volatile liquid carrier mixed with powdered toner as the developing agent. The wet electro-photographic printer using the developing liquid has a better printing quality than the dry electro-photographic printer using the powered toner. Moreover, the wet electro-photographic printer can prevent harm caused by dust generated from the powdered toner, and thus the wet electro-photographic printer has been increasingly used.
FIG. 1 schematically shows a conventional wet electro-photographic printer. The wet electro-photographic printer includes photosensitive media 9, such as OPC drums (Organic Photoconductive Drums), laser scanning units 11, charging rollers 12, developing apparatuses 13, an endless transfer belt 10, such as a photosensitive belt, moving along an endless path, first and second rollers 21 and 22 rotating the transfer belt 10 along the endless path, first transfer rollers 8 transferring an image to the transfer belt 10, a second transfer roller 23 transferring the image to a sheet of printing paper P, a fixing roller 25 fixing the image, and a cleaning blade 51 eliminating an image (developing agents 48) remaining on the transfer belt 10. The enumerated parts form a desired image on the printing paper D by performing an image forming process including charging, light exposing, developing, transferring and fixing processes in turn.
A conventional color printer, such as the conventional wet electro-photographic printer generally includes four laser scanning units 11 for color-printing, and four developing apparatuses 13 respectively containing the four developing agents 48 of black, yellow, cyan and magenta. Each of the developing apparatuses 13 has a developing roller 7 under the photosensitive mediums 9, and a container 6 storing a corresponding one of the developing agents 48.
An operation of the above wet electro-photographic printer is as follows. First of all, when a printing command is generated, an electrically charged layer, that is, an electrostatic latent image corresponding to an image to be printed, is formed on the photosensitive media 9 by the charging rollers 12 and the laser scanning units (LSU) 11. Next, an area where the electrostatic latent image is formed is coated with the developing agents 48 stored in the containers 6 of the developing apparatuses 13 due to an operation of the developing rollers 7 to form a developing agent-image (herein below, it will be just called “an image”) made of toner and a liquid carrier. The image formed on the photosensitive mediums 9 by the developing apparatuses 13 is transferred from the photosensitive media 9 to the transfer belt 10 by first transfer voltage and pressure of the first transfer roller 8 placed inside of the transfer belt 10. The image that has been transferred to the transfer belt 10 is transferred to the printing paper P, which is conveyed through a nip between the transfer belt 10 and the second transfer roller 23 due to second transfer voltage and pressure of the second transfer roller 23 as the transfer belt 10 moves toward the second transfer roller 23 placed at a right side of FIG. 1, which is a side of the second roller 22, while being rotated by the first and second rollers 21 and 22.
The image that has been transferred on the printing paper P is fixed on the printing paper P by the fixing roller 25 and a back-up roller 26, thereby finally forming a desired image.
After the image transferred on the transfer belt 10 is transferred to the printing paper P, the transfer belt 10 moves toward the cleaning blade 51 installed to be in contact with an image forming area of the transfer belt 10 at a side of the first roller 21, that is, a left side of FIG. 1, while being continuously rotated by the first and second rollers 21 and 22. At this point, the developing agent remaining on a surface of the transfer belt 10 is removed from the transfer belt 10 by the cleaning blade 51 to print a next image and is collected in a carrier container 52.
The transfer belt 10 from which the remaining developing agent is removed, repeats the above operation to form and develop a new electrostatic latent image of the next image through the photosensitive media 9, the laser scanning units 11 and the developing apparatuses 13.
Yet, in the conventional printer having the above structure, the image transferred to the transfer belt 10 is directly transferred to the printing paper P by the second transfer roller 23, thus a transfer belt 10 having an elastic layer, such as elastic gum, should be used to transfer the image to the printing paper P having a coarse surface.
Therefore, to prevent the transfer belt 10 from being slacked due to elasticity of the elastic layer, an additional non-elastic layer preventing the elasticity of the elastic layer is formed by coating or attaching on or to the elastic layer. As a result, manufacturing expenses are increased as an additional process is added. Deflection (variation) of a thickness of the elastic layer is generated as the transfer belt 10 becomes thicker, and thus it is difficult to arrange registration of each color.
In addition, since the second transfer roller 23 transfers the image from the transfer belt 10 to the printing paper P using an adequate voltage, for example: DC−2.4 K V, supplied to the printing paper P, the transfer belt 10 should have a thickness from 480 to 680 μm. Therefore, under the above transfer voltage condition, when the transfer belt 10 becomes thicker, an efficiency of the second transfer roller 23 is degraded.
Furthermore, since the elastic layer of the transfer belt 10 is made of a material having elasticity, such as the elastic gum, if the cleaning blade 51 made of a flexible material, such as urethane, and installed at a side of the first roller 21, is operated for a long time while being in close contact with the elastic layer of the transfer belt 10 to remove the developing agent remaining on the surface of the transfer belt 10 after the image is transferred on the printing paper P, then the cleaning blade 51 may cause friction with the elastic layer of the transfer belt 10 so that the cleaning blade 51 is not firmly fixed and caught in the transfer belt 10, or the image forming surface of the transfer belt 10 is damaged.
In a case that the transfer belt 10 is damaged as described above, the image finally formed on the printing paper P is stained with stripes, and noise and vibration are generated for a subsequent cleaning process. Especially, in a case that the vibration is generated to the transfer belt 10, a toner image formed during developing and transferring processes has an additional image of abnormal stripes, such as a jitter, and thus quality of a finally formed image is degraded.
Furthermore, since the conventional printer has the second transfer roller 23 transferring the image from the transfer belt 10 to the printing paper P, which is separated from the fixing apparatuses 25 and 26 fixing the image on the printing paper P through heat and pressure, there is a great possibility of breaking down due to an increase of parts transferring and fixing the image to and/or from the printing paper. Accordingly, reliability of the printer is decreased, and manufacturing expenses increases.